Sandboxing application Firejail blocks sound - I did not install Firejail
Two active sound cards - It's not the case
Brand-new modern sound card - It's not the case
The volume is not right / not all sound channels are working - I installed pavucontrol, see the error image below (pavucontrol.png)

Connection to PulseAudio failed. Autom. retry in 5s
In this case this is likely because PULSE_Server in the Environment/X11 Root Window Properties or default-server in client.conf is misconfigured. This Situation can also arrise when PulseAudio crashed and left stale details in the X11 Root Window. If this is the case, then PulseAudio should autospawn again, or if this is not configured you should run start-pulseaudio-x11 manually.